1. Field of the Invention
The present invention relates to an antireflection film having a low refractive index layer formed of a specific curable composition, and a polarizing plate and an image display device each using the antireflection film. More specifically, the present invention relates to an antireflection film having a low refractive index layer formed of a curable composition comprising a cationic polymerizable compound having a silyl group within the molecular, an inorganic fine particle and a photopolymerization initiator, and a polarizing plate and an image display device each using the antireflection film.
2. Description of the Related Art
In various image display devices such as liquid crystal display device (LCD), plasma display panel (PDP), electroluminescence display (ELD) and cathode ray tube display device (CRT), an antireflection film is disposed on the display surface so as to prevent reduction in the contrast due to reflection of outside light or projection of an image. Accordingly, high transmittance and high physical strength (e.g., scratch resistance) are required of the antireflection film, in addition to high antireflection performance.
The antireflection layer used for the antireflection film has been heretofore provided by forming a single-layer or multilayer thin film. In the case of a single-layer film, a layer (low refractive index layer) having a refractive index lower than that of the substrate may be formed to a thickness of, in terms of the optical film thickness, ¼ the wavelength designed. When more reduction of reflectance is necessary, this may be attained by forming a layer (high refractive index layer) having a refractive index higher than that of the substrate between the substrate and the layer having a low refractive index.
As for the multilayer antireflection film, a multilayer film obtained by laminating transparent metal oxide thin films has been heretofore widely used. The transparent metal oxide thin film is usually formed by a chemical vapor deposition (CVD) process or a physical vapor deposition (PVD) process, particularly by a vacuum vapor deposition process which is a kind of physical vapor deposition process.
The multilayer antireflection film may be formed also by a wet coating process. This process is suitable for mass production and cost reduction as compared with the vacuum vapor deposition process and therefore, formation of an antireflection film by a roll-to-roll method according to the wet coating process is recently more predominating. Along with expanded demand for an antireflection film formed by the wet coating process, a method of producing an antireflection film at a lower cost is strongly demanded.
In the case of producing an antireflection film by the wet coating process, a coating composition prepared by dissolving or dispersing a film-forming composition having a specific refractive index in a solvent is coated on a substrate, then dried and if desired, cured.
As for the formation of a low refractive index layer, there are disclosed a large number of methods of curing a curable resin composition comprising a fluorine-containing compound or an inorganic material as a substance having a low refractive index (see, for example, JP-A-2001-188104 and JP-A-2003-292831).
In particular, for the formation of such a low refractive index layer, an ultraviolet curing method by thermal polymerization using a heat-curable composition or by radical polymerization using a composition comprising a (meth)acrylic resin having an unsaturated double bond is generally employed, and various heat-curable compositions or photoradical polymerizable compositions for the formation of a low refractive index have been developed.
However, in the case of continuously performing heat curing by a roll-to-roll method, a long heat-curing zone is necessary and this requires a large equipment investment for the production apparatus. Furthermore, in order to achieve more reduction of cost and depending on the purpose, elevate the processing speed, the heat-curing zone needs to be extended in proportion to the original length and the equipment investment required becomes larger.
On the other hand, in the case of using ultraviolet curing by radical polymerization, since the radical polymerization is susceptible to curing inhibition by oxygen, the ultraviolet irradiation needs to be performed with a low oxygen concentration. Therefore, in the step of continuously forming an ultraviolet cured resin by the roll-to-roll method, the ultraviolet curing must be performed in an atmosphere having a low oxygen concentration and this requires a special facility having an ultraviolet irradiation zone where the portion except for the film transporting part is sealed from the air. In turn, a large equipment investment becomes necessary.
Particularly, the low refractive layer is a thin film having a thickness of around 0.1 μm and due to its high surface area/volume ratio, this layer is very susceptible to curing inhibition by oxygen. The layer located below (on the support side) the low refractive layer is subjected to ultraviolet irradiation every time a layer is laminated thereon, so that even when curing inhibition by oxygen brings about some influence at the first lamination, curing can be effected at the time of laminating a new layer thereon. However, this effect cannot be expected to occur on the low refractive index layer located as the outermost layer. From these two reasons, oxygen must be more strictly excluded at the curing of the low refractive index layer than at the curing of other layers.
For obtaining a low oxygen concentration, a method of blowing a nitrogen gas into the ultraviolet irradiation zone sealed from the air and expelling oxygen in the zone (nitrogen purging) is employed. In order to form a low refractive index layer having sufficiently high scratch resistance without the effect of the curing inhibition, a large amount of nitrogen must be introduced into the ultraviolet irradiation zone and since a large amount of nitrogen gas needs to be consumed, there arise not only the problem of initial equipment investment but also the problem of running cost.
When the production rate is increased for the purpose of reducing the cost, the amount of oxygen flowed from the air into the ultraviolet irradiation zone along with the transportation of film increases and the oxygen flowed into the zone must be expelled by using a large amount of nitrogen. As a result, the consumption of a nitrogen gas per unit area is increased to greatly decrease the effect of reducing the cost by the elevation of the production rate.
In order to solve the problem of curing inhibition by oxygen in the radical polymerization, a method of forming the low refractive index layer from a cationic polymerizable composition insusceptible to curing inhibition by oxygen in the air may be considered.
A method of forming a low refractive index layer from a cationic polymerizable composition has been proposed (see, for example, JP-A-2004-093947, JP-A-2004-314468 and JP-A-2005-023258). However, the method disclosed in JP-A-2004-314468 is a method of laminating a low refractive index layer directly on a plastic plate by a dipping method, and the method disclosed in JP-A-2005-023258 is a method of laminating a low refractive index layer on a glass plate, where the application to an antireflection film having a hard coat layer as an underlying layer is not disclosed. Particularly, for obtaining an antireflection film having good surface physical properties such as scratch resistance, it is necessary to ensure good adhesion to the underlying layer, which is determined by the relationship with the underlying layer.
Also, in JP-A-2004-093947, a method for the application to an antireflection film having a four-layer structure including a hard coat layer is disclosed, but the application to a two-layer antireflection film which can be produced at a lower cost is not disclosed.
Along with the recent popularization of an antireflection film-mounted image display device such as liquid crystal television, a more inexpensive liquid crystal display device capable of enduring scratching in various environments is required on the market. Accordingly, it is demanded to develop an antireflection film having higher scratch resistance and being producible at a lower cost.